Many Guinea Lynx members have written about the odd things that their cavies have accidentally ingested.

Some guinea pigs tend to nibble on harder plastic items, such as Pigloos and coroplast. Caring owners have spent much time searching through their guinea pigs' droppings, only to discover that the item was, apparently, completely chewed and digested.

Helpful suggestions for those in this situation include encouraging the guinea pig to eat more hay (fiber) and to drink extra water (syringe-feeding water can be beneficial).

There is always a cause for concern if the item swallowed was large, sharp, or otherwise unusual. Signs of distress warrant a visit to the emergency veterinary clinic.

It is wise to check with an experienced cavy veterinarian if there are questions or any behavorial changes. X-rays can be helpful in some cases.

Guinea pig swallowed syringe stopper/plug!Misty, a female, bites off and swallows the syringe stopper while being handfed.
This thread also contains an excellent discussion of the rare instance for using mineral oil (liquid paraffin).